1. Field of the Invention
This invention relates to zoom lenses and, more particularly, to a technique on focusing of the zoom lens.
2. Description of the Related Art
Focusing of the conventional zoom lens is usually performed by moving that lens unit which lies ahead the zoom section, or what is called the "front lens". As the merit of this front lens focusing method mention may be made of the possibility of carrying out focusing and zooming independently of each other. To extend the focusing range to shorter object distances, on the other hand, the total focusing movement must be increased. Further, to admit of that increase, the diameter of the front lens has to be increased. Otherwise, the light beam that proceeds to the margin of the image frame would be vignetted at the full open aperture, so that the sufficient optical performance could not be ensured. This becomes an obstacle in the way to achieve an improvement of the compact form of and a reduction of the weight of the zoom lens. Further, there is limitation even on shortening of the M.O.D. (minimum object distance).
There are previous proposals for solving such problems in, for example, Japanese Laid-Open Patent Applications Nos. Sho 59-74524 and Sho 54-127321. In the former, a focusing technique by moving the variator lens unit constituting the zoom section, and, in the latter, another one by moving the compensator lens unit, are disclosed. Also, Japanese Patent Publication No. Sho 45-12715 discloses a technique of allowing close-up photography to be performed by moving the variator lens unit, the compensator lens unit and, further, a portion of the relay lens unit.
The above-described focusing methods of the Japanese Laid-Open Patent Applications Nos. Sho 59-74524 and Sho 54-127321 are not very advantageous, because the image magnification varies depending even on the object distance, because it is difficult to obtain an increased zoom ratio, and because the total focusing movement increases too rapidly as zooming nears the telephoto end, which calls for a need of, in advance, securing too much dynamic energy and too long space the focusing lens occupies.
Meanwhile, in the Japanese Patent Publication No. Sho 45-12715, the amount of focusing movement imparted into the zoom section is equal to the total focusing movement of the front lens focusing type. Therefore, though the size of the front lens can be reduced to some extent, there is a problem that a very valuable advance in reducing the diameter of the front lens cannot be achieved.
Yet another technique which is also similar to that of Japanese Patent Publication No. Sho 45-12715 is proposed in U.S. Pat. No. 4,054,372.